Magnetic tape recording and reproducing apparatus includes rotary scan devices, in which first and second pairs of diametrically opposite record transducers or heads are positioned within the scanner housing for rotation therewith. In addition, usually, at some given angular displacement, another set of pairs of playback or read heads are positioned in diametrically opposite relationship. For erase before record systems, such rotary scan devices for transferring signals to and/or from a magnetic tape medium often include erase pole tips within the scanner in fixed relation to the record pole tips. This fixed relation is also with respect to the tracks recorded on the magnetic tape, that is the erase pole tip must be in alignment with its associated record pole tip relative to a recorded magnetic tape track.
For editing purposes, these erase pole tips are used to eliminate, or erase, the previously recorded signal prior to re-recording a new signal, this action being an erase operation, followed by a record operation on the same or a subsequent scan with the erase head preceding the record head in time due to its physical positioning within the scanner along with the timing of actuation thereof by the control electronics. Following the erase and record, the signal is then played back on the same or a subsequent scan to verify the accuracy of the erased and re-recorded data. In conventional video tape recording and editing apparatus utilizing rotary scanners, often the record and playback pole tips are arranged in pairs with each head of a pair in proximate relation to the other, to minimize the number of pole tip support structures.
The erase heads, on the other hand, may take any configuration suitable to the format and the machine. In some instances, there is a single erase head per track, in other instances, one head may be utilized to erase two adjacent tracks. Such heads are normally arranged in diametrically opposed relation, with each head having a width sufficient to substantially erase one or more given tracks. In some machines using a single head for erasing two adjacent tracks, the width of the erase head may be slightly greater than the width of the two adjacent tracks, with a portion of the erase head slightly intruding into the two tracks on both sides of the two adjacent tracks being erased. The geometry, in large part, is dictated by the concentration of the flux erasure signal strength, with the signal strength diminishing adjacent the outer edges of the erase head where the head intrudes into the outlying tracks. However, should the erase head be misaligned relative to the two adjacent tracks, excessive erasing may occur on the adjacent tracks. Furthermore, if the erase head is misaligned relative to the associated record head, while it may be possible to position the erase head properly over the previously recorded information by using the control track longitudinal tracking control, the recording would intrude into an adjacent track. Thus it is imperative that the erase head be properly aligned relative to the tracks, and relative to the record head associate therewith.
In addition, track density, that is, the number of tracks per inch of tape, plays a significant part in the accuracy and completeness of the erase process. If the track density is not too high (e.g., early video recording formats were 139 tracks/inch), the manufacturing tolerances which accumulate between the record pole tip locations and the erase pole tip locations within the scanner are small enough (relative to the track size) to be ignored. However, as track densities have increased as with the recent digital formats which are in the range of 650 tracks/inch, the manufacturing tolerances have become a significant portion of the track size and cannot be ignored. Some provision must be made for adjustment of the erase pole tip elevations to ensure they correctly erase the desired track without excessively damaging the adjacent tracks.
It is the object of this invention to provide a simple technique for verifying and/or adjusting the erase pole tip locations relative to the record pole tip locations in a rotary scan device. Furthermore it is the object of this invention to allow this adjustment to be done in the field without the need for ferrofluid development and precision measurement of a tape sample using a microscope.